mobile - Defining resolution or layout size for iphone/ipad devices

Basically the question is divided into three parts -

1. 
   

   What size should I start with on an iPad?

   
   


2. 
   

   At what size do the Graphic Assets need to be passed to the developer?

   
   


3. 
   

   What about the same items on an iPhone?

   
   

With the plethora of devices available --and particularly if I would have to design for an iPhone or iPad-- where should I start detailed design exactly? Should it be for lower res (say 1024 x 768 px for iPad) or should I do it on a higher resolution as for Ipad 2? In case I do it for a higher resolution, should the ppi be at 72 or higher (and how much higher)?

Can I work on 1024 x 768 layout and do the icons scaled up by increasing the ppi? Should I make the detail design at 1024 x 768, leaving the developer to scale it up 2x times through code for the newer iPad versions (which means I share lower version of icon artifacts to developer)? Is there any other means of defining the layout size?

And again, does the same also apply to iPhone layout?